Poly(cyclic)olefin polymers, such as those including norbornene-type repeating unit structures, have shown promise for use in photoresist compositions suitable for exposure at wavelengths such as 193 nm and 157 nm. For example, positive acting (positive tone) photoresists encompassing norbornene-type polymers have shown high dissolution rates after image-wise exposure and post-exposure thermal treatment, as well as superior resistance to dry etch and other typical semiconductor processing steps and acceptably low optical densities at the aforementioned wavelengths.
It is generally known that low molecular weight polymers, such as those used for photoresist compositions, tend to exhibit higher dissolution rates than their higher molecular weight analogs. Unfortunately, it is also known that such low molecular weight materials generally have a higher optical density (OD) than their higher molecular weight analogs. (See, Barclay et al. Macromolecules 1998, 31, 1024 for a discussion of these issues for poly(4-hydroxystyrene), the preferred material for 248 nm photoresists.) As a result, it is often necessary for a person designing such a polymer to target a higher molecular weight than desired for an optimal dissolution rate so that an acceptable OD can be obtained. It follows then that this compromise between optimal dissolution rate and optimal OD results in a photoresist composition that is not optimized for either characteristic.
While optically transparent dissolution rate modifiers (DRMs), a material that can be added to the photoresist composition to enhance the dissolution rate in appropriate areas of the resist and the use of an olefinic chain transfer agent (CTA) during the forming of a polymer can provide for acceptable dissolution rates, DRMs increase both the complexity of the resist composition and its cost, while olefinic CTAs have been found to provide acceptably low molecular weight polymers with higher than desirable ODs.
In U.S. Published Application 2004/0229157, non-olefinic chain transfer agents, such as hydrogen and some alkyl silanes, are described. While both types of CTAs can successfully control the molecular weight of norbornene-type polymers while not resulting in increased optical density, hydrogen's flammability and the need to remove silane residues from the polymer product where alkyl silanes are employed can at times be problematic.
Therefore, it would be desirable to find alternative methods of providing controllably low molecular weight polymers that do not encompass the above-mentioned deficiencies and/or problems while providing for norbornene-type polymers having both controllable molecular weight and appropriately low ODs. Further such alternate methods should not result in an unacceptable reduction in the conversion rate of the polymerization as compared to previously described CTAs. Still further, such alternate methods should not inappropriately increase the complexity of the process or the cost of the resulting polymer.